Thanks to their small size and large index contrast allowing for tight field confinement, optical microfibers are of great interest in nonlinear optics. Their properties have recently been exploited in various devices for supercontinuum generation, pulse compression  and third-harmonic generation . Combining field confinement and field enhancement in a loop or knot resonator can result in low-threshold non-linear microfibre devices, in which pulse shaping effects and bistability can be obtained. Such a behaviour has already been observed at a power level of ten milliwatts with millisecond time response, in the case of thermally-induced non-linearity in silica microfibres . In contrast, the quasi-instantaneous Kerr effect is suitable for the processing of pulses above the GHz range, but requires much higher power, of the order of several hundreds of watts . In the present work, we discuss the conditions under which the Kerr effect dominates over the thermal effect in a silica microfibre resonator, present the experimental set-up we developed to reach these conditions, and the observation of sub-nanosecond pulse shaping that results.
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